This invention relates to a modulated exposure mask for use with conventional photolithographic processes. The mask has a layer of material that absorbs radiation in proportion to its thickness. Further, this absorbing material is shaped so that the amount of radiation absorbed by the mask varies over its surface. A modulated exposure mask according to the invention can be used to create a continuously variable surface topography on a target substrate.
Conventional photolithographic processes are commonly used to make a variety of miniature structures ranging from semiconductor devices to microlens arrays. According to typical conventional photolithographic processes, a target substrate is covered with a photoresistive material (i.e., photoresist). Portions of the photoresist are then exposed to radiation through a mask. The mask embodies a design that completely blocks the radiation from striking selected portions of the photoresist, while fully exposing the other portions of the photoresist. This type of photolithographic mask, that either completely blocks or fully passes incident light, will hereafter be referred to as a xe2x80x9cbinaryxe2x80x9d photolithographic mask.
After the exposure, the exposed portions of the photoresist are removed by a chemical reaction, while the unexposed portions remain (or vice versa). The entire surface, including both the remaining portions of the photoresist material and the uncovered portions of the underlying target substrate, then is etched by a conventional etching process, such as ion milling. This etching process etches the uncovered portions of the target substrate to form the desired structure in the substrate.
One problem with such conventional photolithographic processes, however, is that they will only form a structure with a xe2x80x9cbinaryxe2x80x9d topography. That is, these processes will only form a structure with substantially two distinctive heights or levels: a lower level where the photoresist was removed, and an upper level where the remaining photoresist material covered the substrate. (While imperfections in the photolithographic process, such as diffraction, will cause some variation between the levels at their interfaces, these variations are insubstantial in that they typically cannot be controlled to benefit the structure.)
As the demand for more complex miniature structures has increased, a variety of solutions to this problem have been sought. One well-known proposed solution to this problem is the multi-step photolithographic method discussed in, for example, U.S. Pat. Nos. 4,895,790, 5,161,059, and 5,218,471 to Swanson et al. This process, shown representatively in FIG. 1, uses repeated photolithographic cycles to create multi-level structures.
Discussing this process in more detail, the first process cycle begins with a target substrate 110 covered with a layer of photoresist 120. The photoresist 120 is exposed to ultraviolet light through a first binary mask 130. The exposed portions of the photoresist 120 then are removed, and a dry etch process is used to etch the uncovered portions of the target substrate. This first process cycle produces a two-level structure.
To obtain a multi-level structure, the two-level structure undergoes a second process cycle. The target substrate 110, now having two levels, is covered with a second layer of photoresist 140. Portions of this second layer of photoresist 140 are then exposed to ultraviolet light by a second binary mask 150. The exposed portions of the second photoresist layer 140 are removed, and a second dry etch process etches the uncovered portions of the target substrate 110 to create a structure with four distinct levels. Additional photolithographic cycles can be employed to produce structures with, for example, eight, sixteen, and thirty-two different levels.
This multi-step photolithographic method has several problems, however. First, the process is time consuming and labor intensive, because it requires a conventional photolithographic process to be performed a number of times. Also, each binary mask must be carefully aligned with the substrate so that its exposure exactly corresponds with the structure formed by the previous photolithographic method. Still further, this multi-step photolithographic method can only produce structures with discrete, i.e., xe2x80x9cbinary,xe2x80x9d levels. This method cannot be used to produce a structure with a continuously variable, i.e., xe2x80x9canalog,xe2x80x9d topography, as desired in the art.
Accordingly, other methods have been proposed to produce miniature structures with a continuously variable topography. One type of method, the xe2x80x9cdirect writexe2x80x9d method, uses a high energy density, finely controlled beam to etch material from or deposit material onto the target substrate in order to form the desired structure. Alternatively, such a beam is used to form a desired topography in a layer of photoresist material prior to a conventional etching process. The direct write method can be implemented with a variety of high energy beams, such as a focused ion beam, a high-energy laser beam, and an electron beam. See, for example, U.S. Pat. No. 5,541,411 to Lindquist et al., U.S. Pat. No. 5,148,419 to Gratrix et al., and U.S. Pat. No. 5,811,021 to Zarowin.
The direct write method has problems as well, however. Because each topographical feature must be individually cut, this method is time consuming, and often impractical for creating large devices (i.e., devices greater than a few square millimeters in area). Also, unlike photolithographic processes, devices can only be produced individually by the direct write method, making it impractical for any purpose requiring more than just a few devices. Moreover, the direct write method requires highly expensive equipment, such as a focused ion beam generator or electron beam generator.
Other methods have been proposed to obtain continuously variable topographies with photolithographic processes that employ specialized masks. For example, U.S. Pat. Nos. 5,310,623 and 5,482,800 to Gal teach the use of a xe2x80x9cgray scalexe2x80x9d lithography process. This process employs a binary mask, as with conventional lithographic processes, but the opaque portions of the mask are made up of very small pixels (e.g., 1-100 microns). These pixels are arranged so that the overall image provided by the mask is a gray scale image, like those produced by photocopying machines. That is, these small pixels are arranged so that the percentage of light transmitted by the mask as a whole varies over its surface. With this process, however, the mask must be reimaged by a reduction stepper that inherently has a limited fidelity. Thus, as a practical matter, the application of this gray scale lithography process is typically limited to relatively small devices (e.g., 0.5 inches).
Yet another type of photolithographic method for obtaining a continuously variable or analog topography is the holographic/interferometric type method. This method selectively exposes photoresist to a pattern of radiation caused by interference between multiple coherent beams of radiation. With this method, however, the difficulty in properly setting up the interference pattern limits the surface relief structures that are achievable. Further, only one device can be produced with each holographic/interferometric process, making this method impractical for any use requiring more than just a few devices.
Still another photolithographic method for producing a continuously variable topography is disclosed in U.S. Pat. Nos. 4,567,104, 4,670,366, 4,894,303, 5,078,771, and 5,285,517 to Wu. As described in those patents, this method employs a mask formed from specially doped/implanted high energy beam sensitive (HEBS) glass, which becomes opaque in proportion to its exposure to electron beam radiation. The mask is thus created by exposing portions of the glass to varying amounts of electron beam radiation, so that the opacity of the mask varies over its surface. This method has the drawback, however, that the HEBS glass has a very low sensitivity to the exposing electron beam. It therefore takes a relatively long write time to create a mask, resulting in slow production time and increased cost.
It is therefore an object of the invention to provide a photolithographic process that can easily, quickly and inexpensively produce a structure or device with a continuously variable topography. It is a further object of the invention to provide a photolithographic mask that creates a continuously variable topography with a single photolithographic process step. It is yet an additional object of the invention to provide a method of forming a photolithographic mask that will produce a continuously variable topography with a single photolithographic process step.
According to the invention, a single photolithographic step process produces a structure with a continuously variable (i.e., analog) surface topography. The invention utilizes a modulated exposure mask with a light absorbing material continuously varying in thickness over the primary surface of the mask. This continuously varying thickness of the light absorbing material causes the mask to correspondingly spatially modulate the amount of light it transmits. Thus, in a photolithographic process using the modulated exposure mask, the exposure and subsequent removal of the photoresist vary continuously over the surface of the target substrate.
The photolithographic modulated exposure mask according to the invention can be formed using, for example, the direct write method (e.g., partial electron beam exposure) described above. One preferred method of forming the modulated exposure mask employs a material that absorbs radiation of a desired wavelength, such as ultraviolet light. A layer of this absorbing material, approximately a few microns thick, is positioned on a thicker layer of material transparent to the desired wavelength to form a target substrate. A layer of electron beam resist is then formed over the layer of absorbing material. Next, an electron beam partially exposes the e-beam resist to produce a surface relief structure in the resist. This surface relief structure is then transferred into the thin layer of absorbing material to form the mask according to the invention.